1. Field of the Invention
The present invention relates generally to a rotational magnetic gimbal with an integral magnetic bearing. More particularly, the present invention relates to brushless DC motor technology that provides electromagnetic suspension, using a single electromagnetic actuator to perform both the bearing and rotary torque (motoring) functions.
2. Description of Related Art
Current rotational electric motors employ mechanical bearings in their ends or on the apparatus to which they are providing torque. Recent research in the area of combination motor-bearings has yielded working prototypes of both AC and DC machines. Although some AC machines can provide rotational positioning, more commonly these machines find application where a constant rotating speed is required in spite of load variations. The primary benefit is the potential for high power density and high-speed machines due to the smaller size (length/volume) of combination motor-bearings. Prototypes of AC type motor-bearings have matured to the point of finding applications in a bearingless blood pump and industrial roll.
Of more relevance to high accuracy pointing devices are combination motor-bearings of the DC type, which can provide relatively high torque and levitation forces at zero and low rotational speed. Furthermore, pure magnetic bearings are DC machines, for which a wealth of vibration isolation control algorithms and hardware has been developed. FIGS. 1A and 1B illustrate the present state-of-the-art in design of DC combination motor-bearings.
FIG. 1A shows a brushless permanent magnet (PM) design where the commutation action for motoring is performed using an angular position sensor and digital control logic (not shown), and levitation is performed using radial position sensors and digital control logic (not shown). This design uses PM""s mounted on the surface of the rotor as field poles, and common armature coils for motoring and suspension in the stator. Alternatively, the PM""s can be embedded in the rotor iron to achieve a stronger levitation force than the surface PM design. FIG. 1B shows a reluctance (stepper) motor design that uses a cylindrical, toothed rotor and separate armature coils for motoring and levitation on the stator. The advantage of the stepper motor design is that no angular position sensor is necessarily required. However, in very fine pointing applications, such a sensor may be desirable to overcome torque ripple and cogging common to stepper motor technology. Note that each of these designs is toothed, which creates unwanted detent torque within the actuator and makes accurate pointing more difficult. Evaluation of toothless actuators has been extensively considered by Airex Corporation to eliminate cogging torque and increase peak torque and pointing accuracy for these applications.
While both of the designs shown in FIG. 1A and FIG. 1B offer control of only two translational and one rotational degrees-of-freedom, the principle of operation can be extended to three translational and two rotational degrees-of-freedom.
The most significant design issues for both AC and DC combination motor-bearings are reducing the detrimental effects of magnetic flux cross coupling upon the motoring and bearing control actions, and achieving target force and torque specifications. The main benefit of using combination motor-bearings is reduced size and weight, which follows from common armature coils and flux paths (iron) being used for generation of both motor torque and bearing force. This design efficiency, however, creates potential for significant cross coupling effects that must be carefully treated. The use of common flux paths for motoring and bearing functions must also be carefully designed with regard to maximum torque and force generation capability. Since the maximum flux in a path at any instant is limited by material saturation, the maximum achievable force and torque trade-off against each other during operation. This tradeoff must be considered during the design stage for successful application of the technology.
The present invention provides a rotational magnetic gimbal with an integral magnetic bearing. Brushless DC motor technology provides electromagnetic suspension, using a single electromagnetic actuator to perform both the radial bearing and rotary torque (motoring) functions.
An integrated motor and magnetic bearing consistent with the invention comprises a rotor comprising a plurality of permanent magnets and a stator comprising a plurality of independently controlled coil segments (or sets) magnetically coupled to the permanent magnets, the coil segments comprising a plurality of coil phases. An integrated motor and magnetic bearing may further comprise a first and second radial position sensors, the first radial position sensor disposed in or adjacent to a clearance gap between the rotor and the stator for sensing the position of the rotor with respect to the stator along a first axis, and a second radial position sensor disposed in or adjacent to the clearance gap between the rotor and the stator for sensing the position of the rotor with respect to the stator along a second axis. An integrated motor and magnetic bearing consistent with the invention is capable of providing simultaneously both rotational torque and radial bearing force.
In method form, a method for providing integral electromagnetic motor and bearing functions comprises sensing a first radial position of a rotor, the rotor comprising a plurality of permanent magnets, with respect to a stator along a first axis, the stator comprising a plurality of independently controlled coil segments magnetically coupled to the permanent magnets; and sensing a second radial position of the rotor with respect to the stator along a second axis; and delivering current to at least one coil segment, the amount of current based on at least one sensed position.